Mara and her relations are named in "Dell'Imperadori Constantinopolitani", a manuscript held in the Vatican Library. The document is also known as the "Massarelli manuscript" because it was found in the papers of Angelo Massarelli (1510 - 1566). Masarelli is better known as the general secretary of the Council of Trent, who recorded the daily occurings of the council.
The Massarelli manuscript names her as a daughter of Đurađ Branković and Eirene Kantakouzene. "The Byzantine Lady: Ten Portraits 1250-1500" (1994) by D. M. Nicol questioned her maternity, suggesting Đurađ had a prior marriage to a daughter of John IV of Trebizond. However his theory presented no sources and failed to take into account that John IV was born between 1395 and 1417. He would be unlikely to be a grandparent by the 1410s.
On 11 September, 1429, Durad made a donation to Esphigmenou Monastery at Mount Athos. The charter for the document names his wife Irene and five children. The Masarelli manuscript also names the same five children of Đurađ and Eirene. Other genealogies mention a sixth child, Todor Branković. He could be a child who died young and thus not listed with his siblings.
The oldest sibling listed in the Massarelli document was Grgur Branković. The 1429 document mentions him with the title of Despot. According to "The Late Medieval Balkans, A Critical Survey from the Late Twelfth Century to the Ottoman Conquest" (1994) by J. V. A. Fine, Grgur was appointed governor of territories of southern Serbia associated to the House of Branković. He was reportedly appointed by Murad II of the Ottoman Empire in 1439. In April 1441, Grgur was accused of plotting against Murad and his governorship terminated. He was imprisoned in Amasya and blinded on 8 May, 1441. According to "Monumenta Serbica Spectantia Historiam Serbiae, Bosniae, Ragusii" (1858) by Franc Miklošič, Grgur and his brothers co-signed a charter by which Durad confirmed the privileges of the Republic of Ragusa. The charted was dated to 17 September, 1445. According to the "Europäische Stammtafeln: Stammtafeln zur Geschichte der Europäischen Staaten" (1978) by Detlev Schwennicke, Grgur retired to a monastery under the monastic name "German". According to Fine, Grgur resurfaced in 1458, claiming the succession of the vacant throne of Rascia for himself or his son. The Massarelli manuscript mentioned Grgur as unwed. Later genealogies name his wife as "Jelisaveta". Vuk Grgurević, a son of Grgur, was later a titular Serbian despot (1471-1485). He was possibly an illegitimate.
Mara is mentioned as the second child in the manuscript. Then are listed Stefan Branković and "Cantacuzina", a sister with the Latinized version of their mother's last name. Later genealogies give her name as Katarina. She married Ulrich II of Celje. The last sibling mentioned was Lazar Branković, the youngest of the five.
According to Fine, Mara was betrothed to Murad II in June, 1433. The betrothal was an attempt to prevent an invasion of Serbia from the Ottoman Empire, though periodic Ottoman raids continued. On 4 September, 1435, the marriage took place at Edirne. Her dowry included the districts of Dubočica and Toplica. She had no children of her own and the marriage was possibly never consummated. She was close howeve to her husband's son, Mehmed II.
According to the chronicle of George Sphrantzes, Mara was sent back to her parents when Murad II died. Dating her return to 1451. Sphrantzes records that the widow rejected a marriage proposal by Constantine XI, Byzantine Emperor. Sphrantzes records that when her parents died (in 1456-1457), Mara joined the court of her stepson Mehmed II. According to Nicol, Mara maintained a presence at court but was also offered her own estate at "Ježevo". Nicol identifies Ježevo with the modern settlement of Dafni in Mount Athos. When Mehmed became sultan, she often provided him with advice . Her court at Ježevo included exiled Serbian nobles.
According to Nicol, Mara was joined at "Ježevo" by her sister "Cantacuzina" in 1469. The two ladies acted as intermediaries between Mehmed and the Republic of Venice during the second Turkish–Venetian War (1463 - 1479). . In 1471, Branković personally accompanied a Venetian ambassador to the Porte for negotiations with the Sultan.
She retained her influence over the appointment of leaders of the Orthodox Church, and remained influential during the reign of Mehmed's successor, Bayezid II. Because of her influence, special privileges were offered to the Greek Orthodox Christians of Jerusalem, later extended to the community of Athos Monastery. After the unsuccessful Battle of Vaslui (Moldavia, 1475), Mara remarked that the battle was the worst defeat for the Ottoman Empire.